The present disclosure relates to a bonder head for bonding chips, and particularly, to a bonder head configured to provide a uniform fillet and methods of operating the same.
A semiconductor chip can be bonded to another semiconductor chip or a packaging substrate by employing Controlled Collapse Chip Connection (C4) process. Each C4 ball contacts a C4 pad on the semiconductor chip and another C4 pad on the other semiconductor chip or the packaging substrate. Each C4 pad is a contiguous metal pad typically formed out of the last metal layer of a metal interconnect structure during a semiconductor manufacturing sequence. Each C4 pad is large enough to accommodate the bottom portion of a C4 ball. Typically, an array of C4 balls can be employed to provide input/output (I/O) connections between the semiconductor chip and another semiconductor chip or a packaging substrate.
The C4 connections are susceptible to mechanical stress created by a mismatch in the coefficients of thermal expansion (CTE's) between the semiconductor chip and the other semiconductor chip or the packaging substrate. Such mechanical stress may cause cracks in the C4 balls or the semiconductor chip, causing the semiconductor chip to fail during usage. An underfill material, which easily deforms under stress, i.e., has a low value of Young's modulus, is employed to fill the space around the array of the C4 balls between the semiconductor chip and the other semiconductor chip or the packaging substrate. The underfill material distributes the stress during the thermal expansion or contraction of the semiconductor chip, the array of C4 balls, and the other semiconductor chip or the packaging substrate, thereby preventing cracks in the bonded structure. In order to distribute the stress uniformly, however, it is necessary to have a uniform distribution of the underfill material.
Recently, processes that apply the underfill material on a semiconductor chip or on a packaging substrate have been proposed. These processes enable filling spaces between an array of C4 balls with fewer and/or smaller cavities by applying the underfill material either on the substrate to which C4 balls are attached or to the other substrate to which C4 balls are not attached at the beginning of the bonding process. See, for example, U.S. Pat. No. 6,746,896 to Shi et al. and U.S. Pat. No. 6,902,954 to Shi.
In the cases of wafer level underfill (WLU) or no flow underfill (NFU) known in the art, it is very difficult to get 100% good flip chip joints because the underfill material is easily entrapped between the chip bump and the substrate bump. The non-uniform flow of an underfill material out of chip area is the main reason why the underfill material is entrapped between bumps.
Referring to FIG. 1, a prior art bonded structure employs a pre-applied underfill material, i.e., an underfill material applied before the bonding of the C4 balls. The fillet of the underfill material, i.e., the portion of the underfill material that protrudes laterally beyond the sidewalls of the upper substrate, is not uniform. This non-uniformity is caused by rapid curing of the underfill material, which is exposed to radiation from peripheral portions of a bonding head that extend beyond the area of the underlying substrate. When the underfill material forms the fillet during bonding, the underfill material located in the outer chip area does not flow uniformly due to the direct radiation of heat from a bonder head.
The rapid curing of the underfill material in the fillet reduces the ability of the underfill material to deform at the periphery of the C4 array as well as forming a non-uniform shape for the fillet. Thus, the prior art bonding process often produces irregular fillet shapes and insufficient flow of the underfill material in the outer chip area, and thereby making bad joints (with non-existent or insufficient electrical connection) where the underfill material is entrapped between bump. Referring to FIG. 2, the combination of the non-uniform shape of the fillet and the reduction in the deformability of the underfill material especially at the periphery of the C4 array can result in non-uniform and/or incomplete bonding of the C4 balls as illustrated by the presence of a boundary between a C4 ball and an underlying metallic material.